The present invention relates to the field of non-contact fluid marking devices commonly known as "ink-jet" or "fluid-jet" devices. More particularly, the present invention relates to apparatus and methods for starting the flow of fluid through the orifice plate of a fluid-jet device in a manner such that the fluid filaments will issue from the orifices substantially perpendicular to the orifice plate and without interference with one other.
Fluid-jet printing devices in and of themselves are well known. Typically, prior fluid-jet printing devices provide a linear array of fluid-jet orifices formed in an orifice plate. Filaments or streams of pressurized marking fluid (e.g., ink, dye, textile fabric finishes, etc.) are caused to issue through the orifices which are supplied with fluid from a fluid supply plenum forming part of a fluid distribution bar. An individually-controllable electrostatic charging electrode is disposed downstream of the orifice plate along the so-called "drop formation zone." In accordance with well-known principles of electrostatic induction, the fluid filament is caused to assume an electrical potential opposite in polarity and related in magnitude to the electrical potential of its respective charging electrode. When a droplet of fluid is separated from the filament, the induced electrostatic charge is then trapped on and in the droplet. Thus, subsequent passage of the charged droplet through an electrostatic field having the same polarity as the droplet charge will cause the droplet to be deflected away from a normal droplet path and toward a droplet catching structure. Uncharged droplets, on the other hand, proceed along a normal path and are eventually deposited upon a substrate.
It will be appreciated that the orifice plate has a linear array of very small orifices. The orifices may exceed over up to 1.8 meters or more and have diameters in the range, for example, of about 0.0013-0.01 inch. The orifices may be very closely spaced, along the orifice plate, for example, on the order of 72-200 per inch. With orifices of this size and number (i.e., at 200 to the inch, a 1.8 meter plate has 14173 orifices) disposed along the length of the orifice plate, initial "start-up" of fluid flowing through this extended multi-orificed array has long been a problem. It should be understood that the problem is attendant start-up of the apparatus. Once a filament is formed properly, it rarely degrades. Typically, during start-up of the apparatus, pressure cannot be increased fast enough in the supply of fluid to the plenum chamber above the orifice plate to assure the formation of stable fluid filament streams issuing through the orifice plate in their intended directions, although once properly formed, the filaments are maintained by the steady state fluid pressure. That is, it is desirable that the fluid streams issue from the orifice plate in a direction generally perpendicular to the orifice plate such that each stream does not interfere with adjacent streams or otherwise deviate from the intended straight downward path. When one or more of the fluid streams issue from the orifice plate in other than straight formations, i.e., perpendicular to the orifice plate, a wetting of the orifice plate may occur. Such wetting could deleteriously affect the meniscus of the fluid on the underside of the plate, causing it to disturb the filament formation. This causes substantial difficulties in and prevents obtaining a clear unidirectional curtain of the fluid streams. Typically, this has been overcome in a very tedious manual, orifice-by-orifice, correction process.
Start-up of fluid-jet devices is a well-recognized problem, as shown by the opening two columns of U.S. Pat. No. 4,314,264. One attempted solution to this problem provided for the introduction of fluid into one end of a fluid distribution bar, with air being introduced at the other end of the bar. This was an effort to introduce high pressure within the plenum above the orifice plate cavity to cause the moving wall of fluid to pass down the bar from the fluid entrance to the air entrance in such manner that a sufficient pressure head would develop to cause the issuance of straight fluid streams through the orifices without wetting the plate. It was found, however, that the pressure was inadequate to accomplish that end. Pressurizing the fluid chamber to a greater extent risked damage to the equipment. Also, pressure increases have been found to be particularly ineffective where the geometry of the fluid cavity is not simple and smooth.
One source of the problem is the formation of an air bubble within the confines of the orifice through the orifice plate. In certain orifice plates, the inner and outer surfaces of the plate have inwardly projecting lips or ridges surrounding each orifice. Air bubbles may be formed in the voids between the lips and are trapped during start-up, causing the fluid streams to form at an angle other than perpendicular to the plate. Also, the compressibility of bubbles causes variations in localized pressure, leading to jet instabiliy.
According to the present invention, start-up is accomplished by applying a vacuum to the downstream or fluid filament side of the orifice plate to initially draw air and subsequently the fluid from the fluid supply plenum through the orifice plate. To accomplish this, a vacuum tray is placed under the fluid-jet printing head in a first position sealing the underside of the orifice plate structure outward of the orifices, preferably sealing against the clamping structure for the orifice plate. A vacuum is then drawn in the tray to draw air from the plenum through the orifice plate into the tray. Fluid is then introduced into the fluid distribution bar to fill the plenum and fluid streams issue from the orifice plate under the pressure of the fluid in the plenum and the vacuum pressure applied by the vacuum tray. Since the fluid distribution bar is evacuated prior to the introduction of fluid, bubble formation is less likely than in pressurized situations and any bubbles that are formed are pulled out of the bar by the vacuum.
An important element of the apparatus hereof distributes the vacuum pressure along the orifice plate and locates it just opposite the orifices to provide straight, normal-to-orifice plate filaments. A particular feature of the present invention resides in the use of a tray which has backlighting along one side thereof. Thus, after the vacuum has been applied and the fluid streams started, the tray can be spaced from the orifice plate into a second position such that the fluid streams can be visually observed against the backlight. The tray in its second position is, however, as close to the orifice plate as possible consistent with such visual observation such that any fluid streams issuing from the orifice plate in a non-straight condition (i.e., a condition where the streams lie at an angle other than perpendicular to the orifice plate) will still be caught by the tray. If the fluid streams are issuing properly, i.e., straight, from the orifice plate, and a good start-up condition is obtained, the tray can be lowered to another, or third position. In this position, the tray is located to enable the charging and deflection electrodes to be swung into position along the underside of the orifice plate while the tray continues to catch the fluid streams issuing from the orifice plate. Once the electrodes and catcher structure are in position, voltages may then be applied to the electrodes to obtain a "full catch" condition wherein all of the droplets are charged and deflected to the catcher structure. In this manner, all droplets are diverted to the catcher structure and none of the droplets are discharged into the tray as it lies in its third position. The tray may then be moved to a fourth, out-of-the-way, position such that the substrate, for example, textile materials normally carried on rolls, can be disposed below the orifice plate and printing commenced.
Accordingly, in accordance with the present invention, there is provided, in a fluid-jet printing device of the type having an orifice plate for generating an array of droplet streams for deposition on a substrate, apparatus for starting the flow of fluid droplet streams through the orifice plate comprising means defining a chamber and a margin about the chamber, the chamber-defining means being movable between a first position having its margin sealing about the orifice plate structure for receiving droplet streams flowing through the orifice plate and a second position spaced from the orifice plate. A vacuum is applied to the chamber when the chamber-defining means lies in its first position to draw fluid from the opposite side of the orifice plate through the orifices into the chamber.
Preferably, the chamber-defining means constitutes an elongated tray. Backlighting is provided on the tray for backlighting the area between the tray and the orifice plate when the tray lies in its second position spaced from the orifice plate. This enables visual observation of the fluid droplet streams and the direction of their issuance from the orifice plate while the tray remains closely spaced from the orifice plate in a manner to catch any fluid streams issuing from the orifice plate in directions other than perpendicular thereto.
In a further aspect of the present invention, there is provided a method for starting the flow of fluid droplet streams through the orifices of the orifice plate, including the steps of providing an elongated fluid droplet catching tray in a first position adjacent the underside of the orifice plate structure and in sealing engagement about the orifices of the orifice plate so that all orifices open into the tray, applying a continuous vacuum to the tray to draw fluid from the opposite side of the orifice plate through the orifices into the tray and moving the tray into a second position spaced from the orifice plate.
Accordingly, it is a primary object of the present invention to provide novel and improved apparatus and methods for starting a fluid-jet apparatus in a manner which substantially ensures the initial issuance of fluid droplet streams from the orifice plate in proper direction whereby the desired curtain of droplets may be formed.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.